1. Field of the Invention
This invention relates to the separation of L-glucose and L-mannose from mixtures of the two and other sugar mixtures. Also, this invention relates to the separation by selective adsorption of L-glucose and L-mannose with certain crystalline aluminosilicate zeolitic molecular sieves.
2. Background of the Invention
Artificial sweeteners recently have seen increased use as a replacement for the "natural" sugars, including sucrose and fructose. Such artificial sweeteners have been under continual review for possible adverse long term physiological affects, yet their demand has grown unabated. Accompanying their growth as a commercial area with substantial economic impact has been a renewed emphasis on discovering and supplying new artificial sweeteners, particularly in pure form rather than as mixtures of different products.
The ideal artificial sweetener would be noncaloric, noncariogenic, without detrimental physiological effects, and usable by diabetics. All these requirements would be met if a sweetener were not metabolized by humans and by flora which are found in the mouth and intestinal tract, and if the sweetener were either not absorbed by humans, or absorbed without effect on any internal organ. That is, the ideal sweetener should be excreted in the same form as when ingested. Another desirable feature is that it have bulk properties and texture similar to sucrose so that it can be substituted for table sugar in many formulations. Recently, and perhaps belatedly, attention has turned toward the L-sugars as desirable artificial sweeteners. It has been known since at least 1946 that L-fructose is sweet (M. L. Wolfrom and A. Thompson, J. Am. Chem. Soc., 68, 791,793 (1946)), and since at least 1890 that L-fructose is nonfermentable (E. Fischer, Ber. Deutsch. Chem. Ges., 23 370,389 (1890)), hence not metabolized by microorganisms generally metabolizing D-sugars. A reasonable, although not necessarily correct, inference is that it also is not metabolized by humans. Assuming that L-glucose is a sweet nonmetabolite (Chemtech, Aug. 1979, pp 501,511), it becomes desirable to isolate it from the reaction mixture in which it is normally found and use it as a noncaloric sweetener in many formulations. L-glucose is quite often found in admixture with L-mannose and often with L-mannose in preponderance. More recently Shallenberger and coworkers have demonstrated that many L-sugars have a sweetness comparable to their L-enantiomorphs. Nature, 221, 555 (1969). Cf. R. S. Shallenberger, "The Theory of Sweetness," in Sweeteners and Sweetness, pp 42-50, Edited by G. G. Birch and coworkers; R. S. Shallenberger and T. E. Acree in "The Handbook of Sensory Physiology," Vol. 4, pp 241-5, Edited by L. M. Beider (Springer Verlag, 1971).
Exploitation of the favorable properties of L-sugars is hindered by their relative unavailability. L-glucose for example, is not found to any significant extent in nature. This unavailability has spurred recent efforts in developing commercially feasible methods for preparing L-sugars in amounts necessary for their use as a staple of commerce. Although the preparation of a number of L-sugars is described in U.S. Pat. No. 4,262,032 the focus seems to be on typical laboratory methods wholly unsuited for economical industrial production, in contrast to the process herein.
Glucose can be prepared in several other ways, but usually the product is mixed with mannose. According to Bilik (Czech. Patent No. 149,463 dated July 15, 1973) L-mannose may be epimerized catalytically to L-glucose and L-mannose in 3:1 ratio. Then, L-glucose can be separated by crystallization and the syrup recycled. L-mannose is also produced, along with L-glucose, from L-arabinose by cyanide addition and hydrogenation, according to Arena et al. U.S. Pat. No. 4,581,447. Using L-arabinose at 95% purity or greater, a mixture of L-glucose and L-mannose is produced in almost a 2:1 ratio with about 1% arabinose as an impurity. L-arabinose is one of the few L-sugars available freely in nature, such as from sugar beet pulp and rice hulls. According to U.S. Pat. No. 4,516,566, L-arabinose may be obtained from different sources of cellulose, e.g., beet pulp, wood, along with other saccharides in the product mixtures depending upon the source of cellulose (U.S. Pat. No. 4,516,566 at column I, lines 53-58). Further, U.S. Pat. No. 4,440,855 discloses two other methods for deriving L-glucose and L-mannose from L-arabinose: The Sowden-Fischer conversion (J.A.C.S. Vol. 69 (1947) pp 1963-65) and the Kiliani-Fischer synthesis (Organic Chemistry, Morrison and Boyd (3rd. Ed. 1973) pp 1078-9).
It is known from Sherman et al. U.S. Pat. No. 4,471,114 that mannose and glucose can be separated from a solution of the same by selective adsorption on only certain cation-exchanged type X or type Y zeolitic molecular sieves. Specifically, Ba-exchanged X- or Y-type and Sr-, Na- and Ca-exchanged Y-type zeolites will selectively adsorb mannose thereon. In other words, the separation by this particular zeolite is ion-specific. The nonadsorbed portion is removed from contact with the zeolite. The mannose can be desorbed from the zeolite with a desorbent and recovered.
It is also known from British Patent No. 1,540,556 to separate mannose from glucose with a cationic exchange resin, such as Amberlite XE 200. It has been reported, however, that a two-stage separation, using the identical column in each stage, is required to produce a 98% mannose product. Such a process is inefficient and prohibitively expensive.
The separation of certain specific carbohydrates with dealuminated Y zeolites is also known e.g., Chao et al U.S. Pat. No. 4,394,178, but there is no disclosure therein of my novel process for separating glucose from mannose and, further, the separation disclosed in Chao et al is also ion-specific in that only a very limited number of exchange ions are effective.